Carbonaceous articles are widely used today in both the chemical and metallurgical industries since they exhibit exceptional properties at high temperatures. For example, graphite electrodes are used in electric arc furnaces because they are excellent conductors of electrical current at high furnace temperatures and have good mechanical strength and excellent thermal stability.
However, a major deficiency of carbonaceous articles is that they rapidly oxidize and erode at high temperatures, e.g., above about 500.degree. C. This problem is particularly acute in the case of graphite electrodes for electric arc furnaces. These electrodes must be periodically replaced as they oxidize and erode and this contributes significantly to the final cost of the steel.
It has been proposed to employ oxidation prohibitive coatings on the surfaces of carbonaceous articles, in particular, carbon or graphite electrodes. These protective coatings have been composed of carbides, silicides, oxides and metals, for example. The coatings have been applied to the surfaces of the article using various known methods such as by electric arc, flame spraying or vacuum heating. However, a problem with these protective coatings has been that they cannot withstand severe thermal shock. Such conditions are encountered, for example, when a coated graphite electrode is heated to elevated furnace temperature in excess of about 1200.degree. C. and then rapidly cooled. The protective coating will crack and even spall under these severe conditions and thereby expose the graphite substrate to oxidation. This failure of the protective coating occurs primarily because the coefficient of thermal expansion of the coating does not match that of the graphite substrate or the coating is poorly bonded to the graphite. Consequently, the protective coating and graphite substrate expand and contract at different rates thereby imposing high internal stresses inside the coating and at the coating-graphite interface.
British Pat. No. 1,166,429 discloses a shaped carbon or graphite body provided with an oxidation prohibitive coating, which coating comprises a primary layer applied to the coating and a metallic surfacing layer applied to the primary layer. The primary layer consists of from 90 to 100 weight percent silicon and 0-10 weight percent of one or more of sodium, nitrogen, magnesium, calcium, boron and aluminum. The surfacing layer consists of from 85 to 100 weight percent of aluminum and a total of from 0 to 15 weight percent of one or more metals or compounds including, for example, sodium, magnesium, boron, silicon and phosphorus. Both the primary layer and metallic surfacing layer are applied to the carbon or graphite body by flame spraying techniques. The metallic surfacing layer melts or softens at about the operating temperature of the electrode and seals off any pores that may have developed in the coating during its application. However, a difficulty with this dual layer coating is that the silicon containing primary layer is difficult to apply, it is expensive and bonds to the graphite substrate with difficulty. Furthermore, the low temperature oxidation products in the coating are subject to spallation and erosion.
U.S. Pat. No. 3,140,193 to Kane discloses a graphite electrode having a dual layer protective coating of which the inner layer is made of porous silicon carbide and the outer layer is composed of silicon metal. This protective coating is subject to the same limitation in that the inner layer does not bond to the graphite.
Thus, there is a need in this industry for improved oxidation prohibitive coatings for use on carbonaceous articles, in particular, carbon or graphite electrodes, which avoid the difficulties of the prior art and provides very effective protection against oxidation even under the most severe or abusive conditions, and which is inexpensive and easily adaptable to full scale production.